Abstract

This paper describes an experimental study on field emission characteristics of individual
graphene layers for vacuum nanoelectronics. Graphene layers were prepared by mechanical
exfoliation from a highly oriented pyrolyzed graphite block and placed on an insulating
substrate, with the resulting field emission behavior investigated using a nanomanipulator
operating inside a scanning electron microscope. A pair of tungsten tips controlled
by the nanomanipulator enabled electric connection with the graphene layers without
postfabrication. The maximum emitted current from the graphene layers was 170 nA and
the turn-on voltage was 12.1 V.

Keywords:

Graphene; Field emission; Nanomanipulator; Nanoelectronics

Nano Express

Field emission is a quantum mechanical tunneling phenomenon in which electrons escape
from a solid surface into vacuum, as explained theoretically by R. H. Fowler and L.
Nordheim in 1928. Field emission is widely used in many kinds of vacuum electronic
applications such as flat panel displays, microwave power tubes, electron sources,
and electron-beam lithography. Over the past decade, research groups worldwide have
shown that carbon nanotubes (CNTs) are excellent candidates for electron emission
[1,2]. CNTs possess advantages in aspect ratios, tip radius of curvature, chemical stability,
and mechanical strength. However, issues related to the placement and throughput of
CNT arrays has hampered the development of such arrays for commercial applications.
Here, we use graphene for field emission.

Graphene is a two-dimensional honeycomb-structured single crystal showing ballistic
transport, zero band gap, and electric spin transport characteristics [3-5]. In previous studies, graphene layers were randomly distributed on cathode electrodes
for field emission display applications [6,7]. However, further field emission studies are required using high-quality, planar
graphene structure (e.g. obtained from a highly oriented pyrolyzed graphite (HOPG)
block). In order to understand the fundamental behavior of graphene field emission
and expand its application into vacuum nanoelectronics beyond the field emission display,
the characterization and analysis of field emission from an individual graphene sheet
is necessary.

In this paper, we suggest a new application for graphene in vacuum nanoelectronics.
Figure 1shows a conceptual schematic of a graphene-based triode device. Such a graphene triode
structure can be used as a fundamental unit for vacuum nanoelectronics. The triode
has an in-plane graphene tip (emitter) with the other in-plane electrodes used as
source, drain, and gate on the substrate. Depending on the gate voltage applied, electrons
are emitted from the graphene tip creating an electron current that can be modulated
on and off. To realize this conceptual device, the field emission characteristics
of graphene layers with different thicknesses need to be characterized.

Figure 1. Conceptual schematic view of a graphene-based triode as a fundamental unit for vacuum
nanoelectronics. Depending on the gate voltage applied, electrons are emitted from
the graphene tip creating an electron current that can be modulated on and off

To create the graphene layer for this experimental study, graphene sheets were prepared
by mechanical exfoliation and placed on insulating SiO2 substrate. Figure 2 shows the mechanical exfoliation process of graphene sheets on SiO2. A thermo-curable elastomer, polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning
Co.) film was prepared using a standard recipe on an oxidized Si wafer (see Fig. 2a). The curing temperature and time were 65 °C and 4 h, respectively. After peeling
the film from the wafer, its polished side was scrubbed on a highly oriented pyrolyzed
graphite (HOPG) block (see Fig. 2b, c), and lifted off, transferring graphene layers to the PDMS (Fig. 2d). The exfoliated graphene layers were transferred onto SiO2 thin film by scrubbing the PDMS film and subsequently detaching, leaving behind thin
graphene layers (see Fig. 2e, f). In order to find and evaluate the graphene layers, the thickness of SiO2 layer on Si was set to 300 nm considering optical interference [8].

Figure 2. Fabrication process of graphene sheets using a mechanical exfoliation method. The
graphene sheets are transferred from HOPG block to SiO2layer

A Zyvex Nanomanipulator operating inside a scanning electron microscope (SEM: XL-40
SEM, FEI Co.) was used to measure field emission from individual graphene sheets (Fig.
2). Figure 3 shows the schematic view of the experimental setup for measuring a field emission
current from graphene sheets. In the SEM vacuum chamber, two tungsten tips were located
on the graphene sample; one was contacted directly to the sample and grounded as a
cathode, and the other was placed an arbitrary distance, d, apart from the edge of the sample as the anode. The tungsten tips were connected
to a Keithley semiconductor measurement system via a feed-through in the vacuum chamber
to apply and sense the electric signal for field emission. Figure 4a shows an optical image of the graphene sheets on SiO2 layer. The thickness of the layer was optically measured on 300 nm thick SiO2 layer by using the change of color due to optical interference and transparency [8]. The color change as the number of graphene layers varies is clearly distinguishable.
In Fig. 4a, Cobalt blue, purple, and light purple stand for 8, 4 and 2 nm thicknesses, respectively.
Figure 4b shows an SEM image of graphene sheets with a pair of tungsten tips controlled by
the nanomanipulator.

Figure 3. Schematic view of the experimental setup using a nanomanipulator

After adjusting the position of the tips, a positive potential was applied to the
second tip. The current was then measured during a voltage sweep. Figure 5a shows IE curves of graphene for an arbitrary gap <1 μm. The graphene sheet started to emit
electron current around 20 V and increased exponentially up to 170 nA following the
behavior of the Fowler–Nordheim relationship. The field emission current fluctuated
for applied voltages higher than 33 V. Figure 5b shows FN curves obtained as a result of field emission from a graphene sheet. As shown in
Fig. 5a, the emission current is increased exponentially, and the FN curve shows linear relationship following the field emission behavior. The estimated
turn-on voltages of the tested graphene sheet is 12.1 V, where the slope of FN curve is changed and the linear region (red line) begins as shown in Fig. 5b. In order to estimate the field-enhancement factor, βFN parameters were evaluated by linear fit of the red line as shown in the equations
[9,10].

(1)

(2)

(3)

where I: current, E: electric field (V/d), β: field-enhancement factor, φ: work function, A: area, ħ: reduced Planck constant, and m: electron mass. Assuming the work function of graphene is 5 eV and the gap between
the graphene sheet and the nanomanipulator tip is 1 μm, the estimated field-enhancement
factor, β, is 3519. It is found that the measured field-enhancement factor is comparable with
previous results of graphene film prepared by electrophoresis [7], and the field emission efficiency of graphene is twice as high as other carbon nanomaterials
such as CNT and diamond film [10,11].

From the experimental results, it is found that one can further reduce the voltage
for electron emission as the fabrication process is refined to create a fine emitter
tip from graphene sheets. The field emission properties of graphene need further investigation
in terms of the number of graphene layers and crystallographic arrangement of the
carbon lattice. In the near future, a planar triode device will be studied for next
generation vacuum nanoelectronics.

This field-emitting nanodevice based on the planar form of graphene potentially allows
for top-down CMOS compatible process flows, an advantage for potential industrial
fabrication of electronic devices. For applications where high field emission currents
or low turn-on voltages are required, nanodevices based on graphene would inherently
provide the necessary alignment based on its crystallographic nature.

Acknowledgments

This work has partially been supported by Exchange Student Program by Brain Korea
21, Award No KUK-F1-038-02 made by King Abdullah University of Science and Technology
(KAUST) and National Science Foundation (Major Research Instrumentation Program, Award
No. DMI-0619762).